PROJECT SUMMARY/ABSTRACT Patient derived xenograft (PDX) mouse models are an essential tool for the study of cancer biology, biomarker development, drug screening, and the preclinical evaluation of personalized medicine strategies for many types of cancers. However, xenograft failure rates due to host rejection, even in immunocompromised hosts, is between ~20% and 100% depending on tumor type. Such failure often results in a lost sample and a lost opportunity to obtain clinically relevant data for that patient or tumor type. Emerging evidence suggests that host genetic background underlies much of this variability in xenograft establishment as well as tumor response to drugs. To advance the clinical relevance of the PDX model system, an innovative alternative is to utilize genetic diversity of the host PDX mice as a tool to substantially improve tumor engraftment. The Collaborative Cross (CC) and Diversity Outbred (DO) are complementary sets of recombinant inbred and strains, respectively, that derive from eight parental inbred strains, and combine high genetic diversity with balanced population structures ideal for genetic trait mapping. The CC inbred strains provide a high degree of genetic variability and potentially a genetically tractable platform for sustained and reproducible in vivo PDX studies. DO provides the most genetic variability in outbred genomes with high levels of heterozygosity similar to the human population, and is ideal for characterizing the full range of potential effects of host genetic background variation on tumor cell expansion. To establish a PDX mouse, the host immune system must be suppressed for xenograft acceptance. The challenge in the proposed studies is to suppress the immune system in individual immunocompetent mice with different genetic backgrounds. In Aim 1, the immune system will be suppressed in CC mice by knocking out the Rag1 gene using a novel CRISPR-Cas9 base editing system. In Aim 2, the immune system will be suppressed in genetically unique DO mice using a novel oral immunosuppressant drug. This study is significant because it will provide the genetic diversity necessary for the growth of any type of tumor, expanding the PDX platform to sustain cancers that have previously been impossible to xenograft. This study is conceptually innovative because it will introduce genetic diversity into the PDX platform. This study is technically innovative because it will compromise the immune system in multiple genetically diverse mice by state-of-the-art CRISPR-Cas9 Base Editor technology (Aim 1) or using a novel chemical immunosuppressant in the drinking water (Aim 2). The state-of-the-art Base Editor system in Aim 1 will produce identical mutations in all mice with little or no collateral genetic alterations. Development of these novel platforms has risks that will be rewarded with the generation of PDX engraftment of ?difficult? tumors, enabling a greater understanding of cancer biology and development of novel therapeutics. .